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Stopping power

Paul Hartley | Feb 07, 2003

Air-disc brakes could help farm trucks meet new requirements.

Sometime in the coming months, the National Highway Traffic Safety Administration (NHTSA) is expected to announce new standards for the allowable stopping distances of trucks. The rules won’t take effect for a few years, but vehicle and component manufacturers already are scrambling to meet the anticipated ambitious targets.

Regulators basically want to cut 30% from current medium- and heavy-vehicle stopping distances, which vary by vehicle weight and configuration. At present, lighter commercial vehicles must stop within 215 ft. when traveling 60 mph. Straight trucks weighing more than 33,000 lbs. must stop within 310 ft., and tractor-trailers within 355 ft. The goal of shorter distances is improved highway safety, but meeting that goal will require new, more expensive brake systems and probably fewer options for buyers.

Air-disc brakes. In the near future, Moore says, brakes might become truck-weight specific. Class 4 vehicles (14,001 to 16,000 lbs.) may have one system; Class 5 (16,001 to 19,501 lbs.) may have another; Class 6 (19,501 to 26,000 lbs.) yet another. However, Class 7 (26,001 to 33,000 lbs.) might endure the biggest change. Moore thinks air brakes will be mandated for these heaviest of medium-duty trucks.

“We now have [air brakes] for Class 8 and [hydraulic brakes] for lighter classes,” he says. Although there is some crossover of air brakes, “a definite line exists between medium and heavy duty,” Moore continues. “Soon, though, I think that line will move lower.”

Such a change would surely affect truck-owning farmers, many of whom appreciate the economy and familiarity of hydraulic brakes. A switch to air brakes would mean more, and completely different, components and changes in maintenance requirements.

Of course, this wouldn’t be the first radical shift in medium-duty braking systems. In the mid 1980s, North American truck builders ditched hydraulic drum brakes in favor of hydraulic disc, a conversion that no doubt initially flustered more than a few backyard mechanics. It is unclear whether mandated air brakes for Class 7 trucks would mean a return to drums for that weight group. Air-disc technology is available and, in fact, is popular in Europe. However, it’s as rare as whitewall tires on North American trucks.

The slow adoption of air disc here can be blamed on several factors. Chief among them are price, compatibility and weight, at least when compared with the air-powered drum brakes found on some Class 7 and nearly all Class 8 vehicles.

Prakash Jain, director of international business development for ArvinMeritor, says air disc systems don’t yet offer advantages most truckers are willing to pay extra for. And until sales volumes increase, the prices will remain relatively high. That’s where the government comes in.

“Safety agencies would rather have air-disc brakes than drums on heavy vehicles,” Jain says. “However, they cannot mandate any particular type of brake. They can only mandate brake performance.” Jain and his colleagues believe the upcoming changes in stopping distances will be designed around the known capabilities of air discs. That would nudge the truck and component industries in the direction preferred by regulators. But Jain warns against writing the obituary for drum brakes just yet.

Drum brakes. Last December, ArvinMeritor brought a group of journalists to a research test track in Ohio, where the company demonstrated that drum brakes can outperform discs in a 60-mph emergency stop, if they are fitted with more powerful air chambers and larger friction material.

“We were just trying to show that air disc isn’t the only alternative for better braking,” Jain says. ArvinMeritor manufactures both drum and disc brakes.

Ron Bailey, technical sales manager for air-disc brakes at Bendix, acknowledges that drum brakes can be made more muscular, but he questions the logic and safety of doing so. Larger drum brakes on a front axle can adversely affect driver control, he says. That’s why front brakes on trucks are typically smaller than rear brakes. Furthermore, he contends, a single highway-speed panic stop isn’t necessarily a reasonable gauge of overall brake performance.

Bailey says disc brakes are mechanically preferable to drums because they can deliver consistently shorter stopping distances, in all conditions, without fading or overheating. He blames the truck market’s disinterest in air disc on the soiled reputation of earlier designs. “It’s a shame we didn’t come up with a new name for today’s products, something other that air-disc brakes,” he says. “The current generation of product is [completely different] from its predecessors.”

It is doubtful that a name change alone would boost sales. Disc brakes are still more expensive than drum, and few buyers are willing to pay extra for just greater stopping power — that is, unless they regularly travel over mountainous terrain with heavy loads.

Component manufacturers, though, have been looking for ways to advance brake systems beyond their traditional job description. Traction control, available on numerous vehicles nowadays, is one of the additional roles for brakes. Stability control is another.

ArvinMeritor recently introduced a rollover prevention device that uses a truck’s antilock brake system and its engine’s electronic control module (ECM) to help keep all wheels on the ground, should a driver enter a curve or corner too fast. Called “roll stability control,” the unit is basically a computerized gyroscope that senses unusual lateral forces and then electronically slows the vehicle by decelerating the engine, igniting the compression brake or applying the foundation brakes.

Electronic brakes. Some of these newfangled devices work with today’s standard brake systems — air or hydraulic, drum or disc. Others, some of which are still being developed, require the magic of electronic braking, a technology widely used in Europe but still being tested here.

Electronic braking does for brakes what ECMs do for engines: convert pedal position to an electronic signal, which is then zapped to wheel-end solenoids that apply the precise amount of brake pressure needed. Experts say the system has many benefits. It can automatically ratchet up power on under-performing brakes, resulting in more stable braking and even wear. It also can trigger an engine retarder and dial in the proper amount of braking assistance. The elapsed time between pedal movement and brake actuation is about two-tenths of a second, roughly half the time of air and hydraulic systems. Warning lights immediately alert drivers if a brake becomes disabled. And malfunctions can be quickly and easily diagnosed with a computer and the correct software.

This level of brake wizardry is surely overkill for all but the most techno-savvy truck owners — at least right now.

Technology is only “futuristic” until enough people are employing it. Then it’s considered mainstream. It’s impossible to predict how quickly electronic braking will reach farmers, but that will certainly happen soon after truck builders believe its features offer customers greater value, safety and efficiency.

In the meantime, truck buyers will be adjusting to the new air and hydraulic systems designed to meet the forthcoming changes in NHTSA stopping-distance standards.